[Qemu-devel] iSCSI support for QEMU

[Ronnie Sahlberg]

Please find attached a new version of the patch for iSCSI support.
iSCSI support is automaticallt detected and activated during configure/build
if the libiscsi library is available on the build host.

This library is available at : https://github.com/sahlberg/libiscsi


This new version contains two changes since previous versions
* avoid copying data in the read path and use the api for reading directly
into the application buffers
* use proper task management functions to abort tasks that are cancelled
by qemu


A built in iSCSI initiator has many benefits
* the LUNs used are local to the guest and are not exposed to the host, nor any 
of the processes running on the host.
* it provides very high performance out-of-the-box, compared to open-iscsi
that requires several special flags to iscsi to match libiscsi
* it avoids cache trashing and having two copies of the same data, once in the 
hosts cache and a second copy in the guest. I.e. O_DIRECT like behaviour.
* management of LUNs are much easier for high-end or enterprise users with
very many iscsi devices. You no longer need to expose many hundreds of devices
to the local host and have them pollute the caches.


Performance is acceptable with libiscsi.
Some basic tests thatve been performed show it to be significantly faster
than an out-of-the-box open-iscsi mounted LUN being accessed by default
QEMU i/o options.

Test was performed as booting a Ubuntu 10.04 host, mounted on a 8GB STGT LUN 
exposed across a 1GbE network.
Test was once the system booted, how long would it take to run, inside the 
guest, "sudo time dd if=/dev/sda of=/dev/null bs=1M"
i.e. just have the guest read the 8GB system disk.

QEMU+libiscsi                   : 228 seconds
QEMU, default device options    : 273 seconds
QEMU with cache=none            : 387 seconds
QEMU with cache=none,aio=native : 218 seconds


Please review and consider for inclusion.

regards
ronnie sahlberg



From: Ronnie Sahlberg <address@hidden>
Subject: iSCSI support for QEMU
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